Expandable medical device for delivery of beneficial agent

ABSTRACT

An expandable medical device has a plurality of elongated struts joined together to form a substantially cylindrical device which is expandable from a cylinder having a first diameter to a cylinder having a second diameter. At least one of the plurality of struts includes at least one opening extending at least partially through a thickness of said strut. A beneficial agent is loaded into the opening within the strut in layers to achieve desired temporal release kinetics of the agent. Alternatively, the beneficial agent is loaded in a shape which is configured to achieve the desired agent delivery profile. A wide variety of delivery profiles can be achieved including zero order, pulsatile, increasing, decrease, sinusoidal, and other delivery profiles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. application Ser. No.09/948,989, filed Sep. 7, 2001, which is a continuation-in-part of U.S.application Ser. No. 09/688,092, filed Oct. 16, 2000, which isincorporated herein in its entirety. This application also claimspriority to U.S. Provisional Application Ser. No. 60/314,259, filed Aug.20, 2001 which is incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to tissue-supporting medical devices, andmore particularly to expandable, non-removable devices that areimplanted within a bodily lumen of a living animal or human to supportthe organ and maintain patency, and that can deliver a beneficial agentto the intervention site.

2. Summary of the Related Art

In the past, permanent or biodegradable devices have been developed forimplantation within a body passageway to maintain patency of thepassageway. These devices are typically introduced percutaneously, andtransported transluminally until positioned at a desired location. Thesedevices are then expanded either mechanically, such as by the expansionof a mandrel or balloon positioned inside the device, or expandthemselves by releasing stored energy upon actuation within the body.Once expanded within the lumen, these devices, called stents, becomeencapsulated within the body tissue and remain a permanent implant.

Known stent designs include monofilament wire coil stents (U.S. Pat. No.4,969,458); welded metal cages (U.S. Pat. Nos. 4,733,665 and 4,776,337);and, most prominently, thin-walled metal cylinders with axial slotsformed around the circumference (U.S. Pat. Nos. 4,733,665; 4,739,762;and 4,776,337). Known construction materials for use in stents includepolymers, organic fabrics and biocompatible metals, such as, stainlesssteel, gold, silver, tantalum, titanium, and shape memory alloys such asNitinol.

U.S. Pat. Nos. 4,733,665; 4,739,762; and 4,776,337 disclose expandableand deformable interluminal vascular grafts in the form of thin-walledtubular members with axial slots allowing the members to be expandedradially outwardly into contact with a body passageway. After insertion,the tubular members are mechanically expanded beyond their elastic limitand thus permanently fixed within the body. U.S. Pat. No. 5,545,210discloses a thin-walled tubular stent geometrically similar to thosediscussed above, but constructed of a nickel-titanium shape memory alloy(“Nitinol”), which can be permanently fixed within the body withoutexceeding its elastic limit. All of these stents share a critical designproperty: in each design, the features that undergo permanentdeformation during stent expansion are prismatic, i.e., the crosssections of these features remain constant or change very graduallyalong their entire active length. These prismatic structures are ideallysuited to providing large amounts of elastic deformation beforepermanent deformation commences, which in turn leads to sub-optimaldevice performance in important properties including stent expansionforce, stent recoil, strut element stability, stent securement ondelivery catheters, and radiopacity.

U.S. Pat. No. 6,241,762, which is incorporated herein by reference inits entirety, discloses a non-prismatic stent design which remedies theabove mentioned performance deficiencies of previous stents. Inaddition, preferred embodiments of this patent provide a stent withlarge, non-deforming strut and link elements, which can contain holeswithout compromising the mechanical properties of the strut or linkelements, or the device as a whole. Further, these holes may serve aslarge, protected reservoirs for delivering various beneficial agents tothe device implantation site.

Of the many problems that may be addressed through stent-based localdelivery of beneficial agents, one of the most important is restenosis.Restenosis is a major complication that can arise following vascularinterventions such as angioplasty and the implantation of stents. Simplydefined, restenosis is a wound healing process that reduces the vessellumen diameter by extracellular matrix deposition and vascular smoothmuscle cell proliferation, and which may ultimately result inrenarrowing or even reocclusion of the lumen. Despite the introductionof improved surgical techniques, devices and pharmaceutical agents, theoverall restenosis rate is still reported in the range of 25% to 50%within six to twelve months after an angioplasty procedure. To treatthis condition, additional revascularization procedures are frequentlyrequired, thereby increasing trauma and risk to the patient.

Some of the techniques under development to address the problem ofrestenosis include irradiation of the injury site and the use ofconventional stents to deliver a variety of beneficial or pharmaceuticalagents to the wall of the traumatized vessel. In the latter case, aconventional stent is frequently surface-coated with a beneficial agent(often a drug-impregnated polymer) and implanted at the angioplastysite. Alternatively, an external drug-impregnated polymer sheath ismounted over the stent and co-deployed in the vessel.

While acute outcomes from radiation therapies appeared promisinginitially, long term beneficial outcomes have been limited to reductionin restenosis occurring within a previously implanted stent, so-called‘in-stent’ is restenosis. Radiation therapies have not been effectivefor preventing restenosis in de novo lesions. Polymer sheaths that spanstent struts have also proven problematic in human clinical trials dueto the danger of blocking flow to branch arteries, incomplete appositionof stent struts to arterial walls and other problems. Unacceptably highlevels of MACE (Major Adverse Cardiac Events that include death, heartattack, or the need for a repeat angioplasty or coronary artery bypasssurgery) have resulted in early termination of clinical trials forsheath covered stents.

Conventional stents with surface coatings of various beneficial agents,by contrast, have shown promising early results. U.S. Pat. No.5,716,981, for example, discloses a stent that is surface-coated with acomposition comprising a polymer carrier and paclitaxel (a well-knowncompound that is commonly used in the treatment of cancerous tumors).The patent offers detailed descriptions of methods for coating stentsurfaces, such as spraying and dipping, as well as the desired characterof the coating itself: it should “coat the stent smoothly and evenly”and “provide a uniform, predictable, prolonged release of theanti-angiogenic factor.” Surface coatings, however, can provide littleactual control over the release kinetics of beneficial agents. Thesecoatings are necessarily very thin, typically 5 to 8 microns deep. Thesurface area of the stent, by comparison is very large, so that theentire volume of the beneficial agent has a very short diffusion path todischarge into the surrounding tissue.

Increasing the thickness of the surface coating has the beneficialeffects of improving drug release kinetics including the ability tocontrol drug release and to allow increased drug loading. However, theincreased coating thickness results in increased overall thickness ofthe stent wall. This is undesirable for a number of reasons, includingincreased trauma to the vessel wall during implantation, reduced flowcross-section of the lumen after implantation, and increasedvulnerability of the coating to mechanical failure or damage duringexpansion and implantation. Coating thickness is one of several factorsthat affect the release kinetics of the beneficial agent, andlimitations on thickness thereby limit the range of release rates,durations, and the like that can be achieved.

In addition to sub-optimal release profiles, there are further problemswith surface coated stents. The fixed matrix polymer carriers frequentlyused in the device coatings typically retain approximately 30% of thebeneficial agent in the coating indefinitely. Since these beneficialagents are frequently highly cytotoxic, sub-acute and chronic problemssuch as chronic inflammation, late thrombosis, and late or incompletehealing of the vessel wall may occur. Additionally, the carrier polymersthemselves are often highly inflammatory to the tissue of the vesselwall. On the other hand, use of bio-degradable polymer carriers on stentsurfaces can result in the creation of “virtual spaces”or voids betweenthe stent and tissue of the vessel wall after the polymer carrier hasdegraded, which permits differential motion between the stent andadjacent tissue. Resulting problems include micro-abrasion andinflammation, stent drift, and failure to re-endothelialize the vesselwall.

Another significant problem is that expansion of the stent may stressthe overlying polymeric coating causing the coating to plasticallydeform or even to rupture, which may therefore effect drug releasekinetics or have other untoward effects. Further, expansion of such acoated stent in an atherosclerotic blood vessel will placecircumferential shear forces on the polymeric coating, which may causethe coating to separate from the underlying stent surface. Suchseparation may again have untoward effects including embolization ofcoating fragments causing vascular obstruction.

SUMMARY OF THE INVENTION

In view of the drawbacks of the prior art, it would be advantageous toprovide a stent capable of delivering a relatively large volume of abeneficial agent to a traumatized site in a vessel while avoiding thenumerous problems associated with surface coatings containing beneficialagents, without increasing the effective wall thickness of the stent,and without adversely impacting the mechanical expansion properties ofthe stent.

It would further be advantageous to have such a stent, which alsosignificantly increases the available depth of the beneficial agentreservoir.

It would also be advantageous to have methods of loading variousbeneficial agents or combinations of beneficial agents into these deepreservoirs, which provided control over the temporal release kinetics ofthe agents.

In accordance with one aspect of the invention, an expandable medicaldevice includes a plurality of elongated struts, said plurality ofelongated struts joined together to form a substantially cylindricaldevice which is expandable from a cylinder having a first diameter to acylinder having a second diameter, said plurality of struts each havinga strut width in a circumferential direction and a strut thickness in aradial direction, at least one opening in at least one of the pluralityof struts, and at least one beneficial agent provided in the at leastone opening in a plurality of layers.

In accordance with a further aspect of the present invention, anexpandable medical device includes a plurality of elongated struts, saidplurality of elongated struts joined together to form a substantiallycylindrical device which is expandable from a cylinder having a firstdiameter to a cylinder having a second diameter, said plurality ofstruts each having a strut width in a circumferential direction and astrut thickness in a radial direction, at least one opening in at leastone of the plurality of struts, and at least one beneficial agentprovided in the at least one opening. A shape of the beneficial agent isconfigured to achieve a desired agent delivery profile.

In accordance with another aspect of the present invention, anexpandable medical device for treating cardiac arrhythmias includes anexpandable cylindrical device having a plurality of struts, a pluralityof openings in the plurality of struts, and a chemically ablative agentprovided in the openings. The openings are configured to deliver thechemically ablative agent to tissue surrounding the expandablecylindrical device without permanently trapping any agent in theopenings.

In accordance with an additional aspect of the present invention, anexpandable medical device for treating cardiac arrhythmias includes anexpandable cylindrical device-having a plurality of struts, a pluralityof openings in the plurality of struts, and an anti-arrhythmic drug anda non-biodegradable carrier provided in the openings. The openings areconfigured to deliver the anti-arrhythmic drug to tissue surrounding thecylindrical device over an extended time period.

In accordance with another aspect of the present invention, a method offorming an expandable medical device includes providing an expandablemedical device with a plurality of struts, said plurality of strutsjoined together to form a substantially cylindrical device which isexpandable from a cylinder having a first diameter to a cylinder havinga second diameter, forming at least one opening in at least one of theplurality of struts, and delivering at least one beneficial agent intoin the at least one opening in a plurality of layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe preferred embodiments illustrated in the accompanying drawings, inwhich like elements bear like reference numerals, and wherein:

FIG. 1 is a perspective view of a tissue supporting device in accordancewith a first preferred embodiment of the present invention;

FIG. 2 is an enlarged side view of a portion of the device of FIG. 1;

FIG. 3 is an enlarged side view of a tissue supporting device inaccordance with a further preferred embodiment of the present invention;

FIG. 4 is an enlarged side view of a portion of the stent shown in FIG.3;

FIG. 5 is an enlarged cross section of an opening;

FIG. 6 is an enlarged cross section of an opening illustratingbeneficial agent loaded into the opening;

FIG. 7 is an enlarged cross section of an opening illustrating abeneficial agent loaded into the opening and a thin coating of abeneficial agent;

FIG. 8 is an enlarged cross section of an opening illustrating abeneficial agent loaded into the opening and thin coatings of differentbeneficial agents on different surfaces of the device;

FIG. 9 is an enlarged cross section of an opening illustrating abeneficial agent provided in a plurality of layers;

FIG. 10 is an enlarged cross section of an opening illustrating abeneficial agent and a barrier layer loaded into the opening in layers;

FIG. 11A is an enlarged cross section of an opening illustrating abeneficial agent, a biodegradable carrier, and a barrier layer loadedinto the opening in layers;

FIG. 11B is a graph of the release kinetics of the device of FIG. 11A;

FIG. 12 is an enlarged cross section of an opening illustratingdifferent beneficial agents, carrier, and barrier layers loaded into theopening;

FIG. 13 is an enlarged cross section of an opening illustrating abeneficial agent loaded into the opening in layers of differentconcentrations;

FIG. 14 is an enlarged cross section of an opening illustrating abeneficial agent loaded into the opening in layers of microspheres ofdifferent sizes;

FIG. 15A is an enlarged cross section of a tapered opening illustratinga beneficial agent loaded into the opening;

FIG. 15B is an enlarged cross section of the tapered opening of FIG. 15Awith the beneficial agent partially degraded;

FIG. 15C is a graph of the release kinetics of the device of FIGS. 15Aand 15B;

FIG. 16A is an enlarged cross section of an opening illustrating abeneficial agent loaded into the opening in a shape configured toachieve a desired agent delivery profile;

FIG. 16B is an enlarged cross section of the opening of FIG. 16A withthe beneficial agent partially degraded;

FIG. 16C is a graph of the release kinetics of the device of FIGS. 16Aand 16B;

FIG. 17A is an enlarged cross section of an opening illustrating thebeneficial agent loaded into the opening and a spherical shape;

FIG. 17B is a graph of the release kinetics of the device of FIG. 17A;

FIG. 18A is an enlarged cross section of an opening illustrating aplurality of beneficial agent layers and a barrier layer with an openingfor achieving a desired agent delivery profile;

FIG. 18B is an enlarged cross section of the opening of FIG. 18A withthe agent layers beginning to degraded;

FIG. 18C is an enlarged cross section of the opening of FIG. 18A withthe agent layers further degraded; and

FIG. 19 is an enlarged cross section of an opening illustrating aplurality of cylindrical beneficial agent layers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, a tissue supporting device in accordancewith one preferred embodiment of the present invention is showngenerally by reference numeral 10. The tissue supporting device 10includes a plurality of cylindrical tubes 12 connected by S-shapedbridging elements 14. The bridging elements 14 allow the tissuesupporting device to bend axially when passing through the tortuous pathof the vasculature to the deployment site and allow the device to bendwhen necessary to match the curvature of a vessel wall to be supported.Each of the cylindrical tubes 12 has a plurality of axial slots 16extending from an end surface of the cylindrical tube toward an oppositeend surface.

Formed between the slots 16 is a network of axial struts 18 and links22. The struts 18 and links 22 are provided with openings for receivingand delivering a beneficial agent. As will be described below withrespect to FIGS. 9-17, the beneficial agent is loaded into the openingsin layers or other configurations which provide control over thetemporal release kinetics of the agent.

Each individual strut 18 is preferably linked to the rest of thestructure through a pair of reduced sections 20, one at each end, whichact as stress/strain concentration features. The reduced sections 20 ofthe struts function as hinges in the cylindrical structure. Since thestress/strain concentration features are designed to operate into theplastic deformation range of generally ductile materials, they arereferred to as ductile hinges 20. The ductile hinges 20 are described infurther detail in U.S. Pat. No. 6,241,762, which has been incorporatedherein by reference.

With reference to the drawings and the discussion, the width of anyfeature is defined as its dimension in the circumferential direction ofthe cylinder. The length of any feature is defined as its dimension inthe axial direction of the cylinder. The thickness of any feature isdefined as the wall thickness of the cylinder.

The presence of the ductile hinges 20 allows all of the remainingfeatures in the tissue supporting device to be increased in width or thecircumferentially oriented component of their respective rectangularmoments of inertia—thus greatly increasing the strength and rigidity ofthese features. The net result is that elastic, and then plasticdeformation commence and propagate in the ductile hinges 20 before otherstructural elements of the device undergo any significant elasticdeformation. The force required to expand the tissue supporting device10 becomes a function of the geometry of the ductile hinges 20, ratherthan the device structure as a whole, and arbitrarily small expansionforces can be specified by changing hinge geometry for virtually anymaterial wall thickness. The ability to increase the width and thicknessof the struts 18 and links 22 provides additional area and depth for thebeneficial agent receiving openings.

In the preferred embodiment of FIGS. 1 and 2, it is desirable toincrease the width of the individual struts 18 between the ductilehinges 20 to the maximum width that is geometrically possible for agiven diameter and a given number of struts arrayed around thatdiameter. The only geometric limitation on strut width is the minimumpractical width of the slots 16 which is about 0.002 inches (0.0508 mm)for laser machining. Lateral stiffness of the struts 18 increases as thecube of strut width, so that relatively small increases in strut widthsignificantly increase strut stiffness. The net result of insertingductile hinges 20 and increasing strut width is that the struts 18 nolonger act as flexible leaf springs, but act as essentially rigid beamsbetween the ductile hinges. All radial expansion or compression of thecylindrical tissue supporting device 10 is accommodated by mechanicalstrain in the hinge features 20, and yield in the hinge commences atvery small overall radial expansion or compression.

The ductile hinge 20 illustrated in FIGS. 1 and 2 is exemplary of apreferred structure that will function as a stress/strain concentrator.Many other stress/strain concentrator configurations may also be used asthe ductile hinges in the present invention, as shown and described byway of example in U.S. Pat. No. 6,241,762. The geometric details of thestress/strain concentration features or ductile hinges 20 can be variedgreatly to tailor the exact mechanical expansion properties to thoserequired in a specific application.

Although a tissue supporting device configuration has been illustratedin FIG. 1 which includes ductile hinges, it should be understood thatthe beneficial agent may be contained in openings in stents having avariety of designs including the designs illustrated in U.S. ProvisionalPatent Application Ser. No. 60/314,360, filed on Aug. 20, 2001 and U.S.patent application Ser. No. 09/948,989, filed on Sep. 7, 2001 (AttorneyDocket No. 032304-033), which are incorporated herein by reference. Thepresent invention incorporating beneficial agent openings may also beused with other known stent designs.

As shown in FIGS. 1-4, at least one and more preferably a series ofopenings 24 are formed by laser drilling or any other means known to oneskilled in the art at intervals along the neutral axis of the struts 18.Similarly, at least one and preferably a series of openings 26 areformed at selected locations in the links 22. Although the use ofopenings 24 and 26 in both the struts 18 and links 22 is preferred, itshould be clear to one skilled in the art that openings could be formedin only one of the struts and links. Openings may also be formed in thebridging elements 14. In the embodiment of FIGS. 1 and 2, the openings24, 26 are circular in nature and form cylindrical holes extendingthrough the width of the tissue supporting device 10. It should beapparent to one skilled in the art, however, that openings of anygeometrical shape or configuration could of course be used withoutdeparting from the scope of the present invention. In addition, openingshaving a depth less than the thickness of the device may also be used.

The behavior of the struts 18 in bending is analogous to the behavior ofan I-beam or truss. The outer edge elements 32 of the struts 18, shownin FIG. 2, correspond to the I-beam flange and carry the tensile andcompressive stresses, whereas the inner elements 34 of the struts 18correspond to the web of an I-beam which carries the shear and helps toprevent buckling and wrinkling of the faces. Since most of the bendingload is carried by the outer edge elements 32 of the struts 18, aconcentration of as much material as possible away from the neutral axisresults in the most efficient sections for resisting strut flexure. As aresult, material can be judiciously removed along the axis of the strutso as to form openings 24, 26 without adversely impacting the strengthand rigidity of the strut. Since the struts 18 and links 22 thus formedremain essentially rigid during stent expansion, the openings 24, 26 arealso non-deforming.

The openings 24, 26 in the struts 18 may promote the healing of theintervention site by promoting regrowth of the endothelial cells. Byproviding the openings 24, 26 in the struts, 18, the cross section ofthe strut is effectively reduced without decreasing the strength andintegrity of the strut, as described above. As a result, the overalldistance across which endothelial cell regrowth must occur is alsoreduced to approximately 0.0025-0.0035 inches, which is approximatelyone-half of the thickness of a conventional stent. It is furtherbelieved that during insertion of the expandable medical device, cellsfrom the endothelial layer may be scraped from the inner wall of thevessel by the openings 24, 26 and remain therein after implantation. Thepresence of such endothelial cells would thus provide a basis for thehealing of the vessel wall.

The openings 24, 26 are loaded with an agent, most preferably abeneficial agent, for delivery to the vessel wall which the tissuesupporting device 10 is supporting.

The terms “agent” and “beneficial agent” as used herein are intended tohave their broadest possible interpretation and are used to include anytherapeutic agent or drug, as well as inactive agents such as barrierlayers or carrier layers. The terms “drug” and “therapeutic agent” areused interchangeably to refer to any therapeutically active substancethat is delivered to a bodily conduit of a living being to produce adesired, usually beneficial, effect. The present invention isparticularly well suited for the delivery of antiproliferatives(anti-restenosis agents) such as paclitaxel and rapamycin for example,and antithrombins such as heparin, for example.

The beneficial agents used in the present invention include classicalsmall molecular weight therapeutic agents commonly referred to as drugsincluding all classes of action as exemplified by, but not limited to:antiproliferatives, antithrombins, antiplatelet, antilipid,anti-inflammatory, and anti-angiogenic, vitamins, ACE inhibitors,vasoactive substances, antimitotics, metello-proteinase inhibitors, NOdonors, estradiols, anti-sclerosing agents, alone or in combination.Beneficial agent also includes larger molecular weight substances withdrug like effects on target tissue sometimes called biologic agentsincluding but not limited to: peptides, lipids, protein drugs, enzymes,oligonucleotides, ribozymes, genetic material, prions, virus, bacteria,and eucaryotic cells such as endothelial cells, monocyte/macrophages orvascular smooth muscle cells to name but a few examples. Otherbeneficial agents may include but not be limited to physical agents suchas microspheres, microbubbles, liposomes, radioactive isotopes, oragents activated by some other form of energy such as light orultrasonic energy, or by other circulating molecules that can besystemically administered.

The embodiment of the invention shown in FIGS. 1 and 2 can be furtherrefined by using Finite Element Analysis and other techniques tooptimize the deployment of the beneficial agent within the openings ofthe struts and links. Basically, the shape and location of the openings24, 26 can be modified to maximize the volume of the voids whilepreserving the relatively high strength and rigidity of the struts 18with respect to the ductile hinges 20.

FIG. 3 illustrates a further preferred embodiment of the presentinvention, wherein like reference numerals have been used to indicatelike components. The tissue supporting device 100 includes a pluralityof cylindrical tubes 12 connected by S-shaped bridging elements 14. Eachof the cylindrical tubes 12 has a plurality of axial slots 16 extendingfrom an end surface of the cylindrical tube toward an opposite endsurface. Formed between the slots 16 is a network of axial struts 18 andlinks 22. Each individual strut 18 is linked to the rest of thestructure through a pair of ductile hinges 20, one at each end, whichact as stress/strain concentration features. Each of the ductile hinges20 is formed between an arc surface 28 and a concave notch surface 29.

At intervals along the neutral axis of the struts 18, at least one andmore preferably a series of openings 24′ are formed by laser drilling orany other means known to one skilled in the art. Similarly, at least oneand preferably a series of openings 26′ are formed at selected locationsin the links 22. Although the use of openings 24′, 26′ in both thestruts 18 and links 22 is preferred, it should be clear to one skilledin the art that openings could be formed in only one of the struts andlinks. In the illustrated embodiment, the openings 24′ in the struts 18are generally rectangular whereas the openings 26′ in the links 22 arepolygonal. It should be apparent to one skilled in the art, however,that openings of any geometrical shape or configuration could of coursebe used, and that the shape of openings 24, 24′ may be the same ordifferent from the shape of openings 26, 26′, without departing from thescope of the present invention. As described in detail above, theopenings 24′, 26′ may be loaded with an agent, most preferably abeneficial agent, for delivery to the vessel in which the tissue supportdevice 100 is deployed. Although the openings 24′, 26′ are preferablythrough openings, they may also be recesses extending only partiallythrough the thickness of the struts and links.

The relatively large, protected openings 24, 24′, 26, 26′, as describedabove, make the expandable medical device of the present inventionparticularly suitable for delivering agents having more esoteric largermolecules or genetic or cellular agents, such as, for example, proteindrugs, enzymes, antibodies, antisense oligonucleotides, ribozymes,gene/vector constructs, and cells (including but not limited to culturesof a patient's own endothelial cells). Many of these types of agents arebiodegradable or fragile, have a very short or no shelf life, must beprepared at the time of use, or cannot be pre-loaded into deliverydevices such as stents during the manufacture thereof for some otherreason. The large through-openings in the expandable device of thepresent invention form protected areas or receptors to facilitate theloading of such an agent either at the time of use or prior to use, andto protect the agent from abrasion and extrusion during delivery andimplantation.

The volume of beneficial agent that can be delivered using throughopenings is about 3 to 10 times greater than the volume of a 5 microncoating covering a stent with the same stent/vessel wall coverage ratio.This much larger beneficial agent capacity provides several advantages.The larger capacity can be used to deliver multi-drug combinations, eachwith independent release profiles, for improved efficacy. Also, largercapacity can be used to provide larger quantities of less aggressivedrugs and to achieve clinical efficacy without the undesirableside-effects of more potent drugs, such as retarded healing of theendothelial layer.

Through openings also decrease the surface area of the beneficial agentbearing compounds to which the vessel wall surface is exposed. Fortypical devices with beneficial agent openings, this exposure decreasesby a factors ranging from about 6:1 to 8:1, by comparison with surfacecoated stents. This dramatically reduces the exposure of vessel walltissue to polymer carriers and other agents that can cause inflammation,while simultaneously increasing the quantity of beneficial agentdelivered, and improving control of release kinetics.

FIG. 4 shows an enlarged view of one of the struts 18 of device 100disposed between a pair of ductile hinges 20 having a plurality ofopenings 24′. FIG. 5 illustrates a cross section of one of the openings24′ shown in FIG. 4. FIG. 6 illustrates the same cross section when abeneficial agent 36 has been loaded into the opening 24′ of the strut18. Optionally, after loading the opening 24′ and/or the opening 26′with a beneficial agent 36, the entire exterior surface of the stent canbe coated with a thin layer of a beneficial agent 38, which may be thesame as or different from the beneficial agent 36, as schematicallyshown in FIG. 7. Still further, another variation of the presentinvention would coat the outwardly facing surfaces of the stent with afirst beneficial agent 38 while coating the inwardly facing surfaces ofthe stent with a different beneficial agent 39, as illustrated in FIG.8. The inwardly facing surface of the stent would be defined as at leastthe surface of the stent which, after expansion, forms the inner passageof the vessel. The outwardly facing surface of the stent would bedefined as at least the surface of the stent which, after expansion, isin contact with and directly supports the inner wall of the vessel. Thebeneficial agent 39 coated on the inner surfaces may be a barrier layerwhich prevents the beneficial agent 36 from passing into the lumen ofthe blood vessel and being washed away in the blood stream.

FIG. 9 shows a cross section of an opening 24 in which one or morebeneficial agents have been loaded into the opening 24 in discretelayers 50. One method of creating such layers is to deliver a solutioncomprising beneficial agent, polymer carrier, and a solvent into theopening and evaporating the solvent to create a thin solid layer ofbeneficial agent in the carrier. Other methods of delivering thebeneficial agent can also be used to create layers. According to anothermethod for creating layers, a beneficial agent may be loaded into theopenings alone if the agent is structurally viable without the need fora carrier. The process can then be repeated until each opening ispartially or entirely filled.

In a typical embodiment, the total depth of the opening 24 is about 125to about 140 microns, and the typical layer thickness would be about 2to about 50 microns, preferably about 12 microns. Each typical layer isthus individually about twice as thick as the typical coating applied tosurface-coated stents. There would be at least two and preferably aboutten to twelve such layers in a typical opening, with a total beneficialagent thickness about 25 to 28 times greater than a typical surfacecoating. According to one preferred embodiment of the present invention,the openings have an area of at least 5×10⁻⁶ square inches, andpreferably at least 7×10⁻⁶ square inches.

Since each layer is created independently, individual chemicalcompositions and pharmacokinetic properties can be imparted to eachlayer. Numerous useful arrangements of such layers can be formed, someof which will be described below. Each of the layers may include one ormore agents in the same or different proportions from layer to layer.The layers may be solid, porous, or filled with other drugs orexcipients.

FIG. 9 shows the simplest arrangement of layers including identicallayers 50 that together form a uniform, homogeneous distribution ofbeneficial agent. If the carrier polymer were comprised of abiodegradable material, then erosion of the beneficial agent containingcarrier would occur on both faces of the opening at the same time, andbeneficial agent would be released at an approximately linear rate overtime corresponding to the erosion rate of the carrier. This linear orconstant release rate is referred to as a zero order delivery profile.Use of biodegradable carriers in combination with through openings isespecially useful, to guarantee 100% discharge of the beneficial agentwithin a desired time without creating virtual spaces or voids betweenthe radially outermost surface of the stent and tissue of the vesselwall. When the biodegradable material in the through openings isremoved, the openings may provide a communication between thestrut-covered vessel wall and the blood stream. Such communication mayaccelerate vessel healing and allow the ingrowth of cells andextracellular components that more thoroughly lock the stent in contactwith the vessel wall. Alternatively, some through-openings may be loadedwith beneficial agent while others are left unloaded. The unloaded holescould provide an immediate nidus for the ingrowth of cells andextracellular components to lock the stent into place, while loadedopenings dispense the beneficial agent.

The advantage of complete erosion using the through openings oversurface coated stents opens up new possibilities for stent-basedtherapies. In the treatment of cardiac arrhythmias, such as atrialfibrillation both sustained and paroxysmal, sustained ventriculartachycardia, super ventricular tachycardia including reentrant andectopic, and sinus tachycardia, a number of techniques under developmentattempt to ablate tissue in the pulmonary veins or some other criticallocation using various energy sources, e.g. microwaves, generallyreferred to as radio-frequency ablation, to create a barrier to thepropagation of undesired electrical signals in the form of scar tissue.These techniques have proven difficult to control accurately. A stentbased therapy using through openings, biodegradable carriers, andassociated techniques described herein could be used to deliver achemically ablative agent in a specific, precise pattern to a specificarea for treatment of atrial fibrillation, while guaranteeing that noneof the inherently cytotoxic ablating agent could be permanently trappedin contact with the tissue of the vessel wall.

If, on the other hand, the goal of a particular therapy is to provide along term effect, beneficial agents located in openings provide anequally dramatic advantage over surface coated devices. In this case, acomposition comprising a beneficial agent and a non-biodegradablecarrier would be loaded into the through openings, preferably incombination with a diffusion barrier layer as described below. Tocontinue the cardiac arrhythmias example, it might be desirable tointroduce a long-term anti-arrhythmic drug near the ostia of thepulmonary veins or some other critical location. The transient diffusionbehavior of a beneficial agent through a non-biodegradable carriermatrix can be$\frac{\partial C_{x}}{\partial t} = {\frac{\partial}{\partial x}\left\lbrack {D\frac{\partial C_{x}}{\partial x}} \right\rbrack}$generally described by Fick's second law:

Where C is the concentration of beneficial agent at cross section x, xis either the thickness of a surface coating or depth of a throughopening, D is the diffusion coefficient and t is time. The solution ofthis partial differential equation for a through opening with a barrierlayer will have the form of a normalized

probability integral or Gaussian Error Function, the argument of whichwill $\frac{x}{2\sqrt{Dt}}$contain the term

To compare the time intervals over which a given level of therapy can besustained for surface coatings vs. through openings, we can use Fick'sSecond Law to compare the times required to achieve equal concentrationsat the most inward surfaces of the coating and opening respectively,i.e. the values of x and t for which the arguments of $\begin{matrix}{\frac{x_{1}}{2\sqrt{{Dt}_{1}}} = \left. \frac{x_{2}}{2\sqrt{{Dt}_{2}}}\Rightarrow\frac{x_{1}^{2}}{x_{2}^{2}} \right.} \\{= \frac{t_{1}}{t_{2}}}\end{matrix}$the Error Function are equal:

The ratio of diffusion times to achieve comparable concentrations thusvaries as the square of the ratio of depths. A typical opening depth isabout 140 microns while a typical coating thickness is about 5 micron;the square of this ratio is 784, meaning that the effective duration oftherapy for through openings is potentially almost three orders ofmagnitude greater for through openings than for surface coatings of thesame composition. The inherent non-linearity of such release profilescan in part be compensated for in the case of through openings, but notin thin surface coatings, by varying the beneficial agent concentrationof layers in a through opening as described below. It will be recalledthat, in addition to this great advantage in beneficial agent deliveryduration, through openings are capable of delivering a 3 to 10 timesgreater quantity of beneficial agent, providing a decisive overalladvantage in sustained therapies. The diffusion example aboveillustrates the general relationship between depth and diffusion timethat is characteristic of a wider class of solid state transportmechanisms.

Beneficial agent that is released to the radially innermost or inwardlyfacing surface known as the lumen facing surface of an expanded devicemay be rapidly carried away from the targeted area, for example by thebloodstream, and thus lost. Up to half of the total agent loaded in suchsituations may have no therapeutic effect due to being carried away bythe bloodstream. This is probably the case for all surface coated stentsas well as the through opening device of FIG. 9.

FIG. 10 shows a device in which the first layer 52 is loaded into athrough opening 24 such that the inner surface of the layer issubstantially co-planar with the inwardlyfacing surface 54 of thecylindrical device. The first layer 52 is comprised of a material calleda barrier material which blocks or retards biodegradation of subsequentlayers in the inwardly facing direction toward the vessel lumen, and/orblocks or retards diffusion of the beneficial agent in that direction.Biodegradation of other layers or beneficial agent diffusion can thenproceed only in the direction of the outwardly facing surface 56 of thedevice, which is in direct contact with the targeted tissue of thevessel wall. The barrier layer 52 may also function to prevent hydrationof inner layers of beneficial agent and thus prevent swelling of theinner layers when such layers are formed of hygroscopic materials. Thebarrier layer 52 may further be comprised of a biodegradable materialthat degrades at a much slower rate than the biodegradable material inthe other layers, so that the opening will eventually be entirelycleared. Providing a barrier layer 52 in the most inwardly facingsurface of a through-opening thus guarantees that the entire load ofbeneficial agent is delivered to the target area in the vessel wall. Itshould be noted that providing a barrier layer on the inwardly facingsurface of a surface-coated stent without openings does not have thesame effect; since the beneficial agent in such a coating cannot migratethrough the metal stent to the target area on the outer surface, itsimply remains trapped on the inner diameter of the device, again havingno therapeutic effect.

Barrier layers can be used to control beneficial agent release kineticsin more sophisticated ways. A barrier layer 52 with a pre-determineddegradation time could be used to deliberately terminate the beneficialagent therapy at a pre-determined time, by exposing the underlyinglayers to more rapid bio-degradation from both sides. Barrier layers canalso be formulated to be activated by a separate, systemically appliedagent. Such systemically applied agent could change the porosity of thebarrier layer and/or change the rate of bio-degradation of the barrierlayer or the bulk beneficial agent carrier. In each case, release of thebeneficial agent could be activated by the physician at will by deliveryof the systemically applied agent. A further embodiment of physicianactivated therapy would utilize a beneficial agent encapsulated inmicro-bubbles and loaded into device openings. Application of ultrasonicenergy from an exterior of the body could be used to collapse thebubbles at a desired time, releasing the beneficial agent to diffuse tothe outwardly facing surface of the reservoirs. These activationtechniques can be used in conjunction with the release kinetics controltechniques described herein to achieve a desired drug release profilethat can be activated and/or terminated at selectable points in time.

FIG. 11A shows an arrangement of layers provided in a through opening inwhich layers 50 of a beneficial agent in a biodegradable carriermaterial, are alternated with layers 58 of the biodegradable carriermaterial alone, with no active agent loaded, and a barrier layer 52 isprovided at the inwardly facing surface. As shown in the releasekinetics plot of FIG. 11B, such an arrangement releases beneficial agentin three programmable bursts or waves achieving a stepped or pulsatiledelivery profile. The use of carrier material layers without activeagent creates the potential for synchronization of drug release withcellular biochemical processes for enhanced efficacy.

Alternatively, different layers could be comprised of differentbeneficial agents altogether, creating the ability to release differentbeneficial agents at different points in time, as shown in FIG. 12. Forexample, in FIG. 12, a layer 60 of anti-thrombotic agent could bedeposited at the inwardly facing surface of the stent, followed by abarrier layer 52 and alternating layers of anti-proliferatives 62 andanti-inflamatories 64. This configuration could provide an initialrelease of anti-thrombotic agent into the bloodstream whilesimultaneously providing a gradual release of anti-proliferativesinterspersed with programmed bursts of anti-inflammatory agents to thevessel wall. The configurations of these layers can be designed toachieve the agent delivery bursts at particular points in timecoordinated with the body's various natural healing processes.

A further alternative is illustrated in FIG. 13. Here the concentrationof the same beneficial agent is varied from layer to layer, creating theability to generate release profiles of arbitrary shape. Progressivelyincreasing the concentration of agent in the layers 66 with increasingdistance from the outwardly facing surface 56, for example, produces arelease profile with a progressively increasing release rate, whichwould be impossible to produce in a thin surface coating.

Another general method for controlling beneficial agent release kineticsis to alter the beneficial agent flux by changing the surface area ofdrug elution sources as a function of time. This follows from Fick'sFirst Law, which states that the instantaneous molecular flux isproportional to surface area, among other factors: $\begin{matrix}{J = \left. {D\frac{\partial C}{\partial x}}\Rightarrow\frac{\partial N}{\partial t} \right.} \\{= {{AD}\frac{\partial c}{\partial x}}}\end{matrix}$

Where ∂N/∂t is the number of molecules per unit time, A is theinstantaneous drug eluting surface area, D is the diffusivity, and C isthe concentration. The drug eluting surface area of a surface coatedstent is simply the surface area of the stent itself. Since this area isfixed, this method of controlling release kinetics is not available tosurface coated devices. Through openings, however, present severalpossibilities for varying surface area as a function of time.

In the embodiment of FIG. 14, beneficial agent is provided in theopenings 24 in the form of microspheres, particles or the like.Individual layers 70 can then be created that contain these particles.Further, the particle size can be varied from layer to layer. For agiven layer volume, smaller particle sizes increase the total particlesurface area in that layer, which has the effect of varying the totalsurface area of the beneficial agent from layer to layer. Since the fluxof drug molecules is proportional to surface area, the total drug fluxcan be adjusted from layer to layer by changing the particle size, andthe net effect is control of release kinetics by varying particle sizeswithin layers.

A second general method for varying drug eluting surface area as afunction of time is to change the shape or cross-sectional area of thedrug-bearing element along the axis of the opening. FIG. 15A shows anopening 70 having a conical shape cut into the material of the stentitself. The opening 70 may then be filled with beneficial agent 72 inlayers as described above or in another manner. In this embodiment, abarrier layer 74 may be provided on the inwardly facing side of theopening 70 to prevent the beneficial agent 72 from passing into theblood stream. In this example, the drug eluting surface area A, wouldcontinuously diminish (from FIG. 15A to FIG. 15B) as the bio-degradablecarrier material erodes, yielding the elution pattern of FIG. 15C.

FIG. 16A shows a simple cylindrical through-opening 80 in which apreformed, inverted cone 82 of beneficial agent has been inserted. Therest of the through opening 80 is then back-filled with a biodegradablesubstance 84 with a much slower rate of degradation or anon-biodegradable substance, and the inwardly facing opening of thethrough opening is sealed with a barrier layer 86. This technique yieldsthe opposite behavior to the previous example. The drug-eluting surfacearea A_(t) continuously increases with time between FIG. 16A and 16B,yielding the elution pattern of FIG. 16C.

The changing cross section openings 70 of FIG. 15A and thenon-biodegradable backfilling techniques of FIG. 16A may be combinedwith any of the layered agent embodiments of FIGS. 9-14 to achievedesired release profiles. For example, the embodiment of FIG. 15A mayuse the varying agent concentration layers of FIG. 13 to more accuratelytailor a release curve to a desired profile.

The process of preforming the beneficial agent plug 82 to a specialshape, inserting in a through opening, and back-filling with a secondmaterial can yield more complex release kinetics as well. FIG. 17A showsa through opening 90 in which a spherical beneficial agent plug 92 hasbeen inserted. The resulting biodegradation of the sphere, in which thecross sectional surface area varies as a sinusoidal function of depth,produces a flux density which is roughly a sinusoidal function of time,FIG. 17B. Other results are of course possible with other profiles, butnone of these more complex behaviors could be generated in a thin,fixed-area surface coating.

An alternative embodiment of FIGS. 18A-18C use a barrier layer 52′ withan opening 96 to achieve the increasing agent release profile of FIG.16C. As shown in FIG. 18A, the opening 24 is provided with an innerbarrier layer 52 and multiple beneficial agent layers 50 as in theembodiment of FIG. 10. An additional outer barrier layer 52′ is providedwith a small hole 96 for delivery of the agent to the vessel wall. Asshown in FIGS. 18B and 18C, the beneficial agent containing layers 50degrade in a hemispherical pattern resulting in increasing surface areafor agent delivery over time and thus, an increasing agent releaseprofile.

FIG. 19 illustrates an alternative embodiment in which an opening in thetissue supporting device is loaded with cylindrical layers of beneficialagent. According to one method of forming the device of FIG. 19, theentire device is coated with sequential layers 100, 102, 104, 106 ofbeneficial agent. The interior surface 54 and exterior surface 56 of thedevice are then stripped to remove the beneficial agent on thesesurfaces leaving the cylindrical layers of beneficial agent in theopenings. In this embodiment, a central opening remains after thecoating layers have been deposited which allows communication betweenthe outer surface 56 and inner surface 54 of the tissue supportingdevice.

In the embodiment of FIG. 19, the cylindrical layers are erodedsequentially. This can be used for pulsatile delivery of differentbeneficial agents, delivery of different concentrations of beneficialagents, or delivery of the same agent. As shown in FIG. 19, the ends ofthe cylindrical layers 100, 102, 104, 106 are exposed. This results in alow level of erosion of the underlying layers during erosion of anexposed layer. Alternatively, the ends of the cylindrical layers may becovered by a barrier layer to prevent this low level continuous erosion.Erosion rates of the cylindrical layers may be further controlled bycontouring the surfaces of the layers. For example, a ribbed orstar-shaped pattern may be provided on the radially inner layers toprovide a uniform surface area or uniform erosion rate between theradially inner layers and the radially outer layers. Contouring of thesurfaces of layers may also be used in other embodiments to provide anadditional variable for controlling the erosion rates.

While the invention has been described in detail with reference to thepreferred embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made and equivalentsemployed, without departing from the present invention.

1. An expandable medical device for treating cardiac arrhythmias, thedevice comprising: an expandable cylindrical device having a pluralityof struts; a plurality of openings in the plurality of struts; and achemically ablative agent provided in the openings, wherein the openingsare configured to deliver the chemically ablative agent to tissuesurrounding the expandable cylindrical device without permanentlytrapping any agent in the openings.
 2. The expandable medical device ofclaim 1, further comprising a biodegradable barrier layer provided inthe opening substantially adjacent an innermost surface of thecylindrical device.